The present invention is directed to flow meters and in particular, float-type flow meters, used to measure the flow of air, industrial gasses or other fluids. In general, flow meters may be used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. In float-type flow meters, a float is mounted for movement within a tapered passage in the body of the meter. The float will rise in proportion to the rate of fluid flow. Typically, the float carries a rod which extends downwardly into an elongated guard that houses a sight glass. An indicator mounted on the lower end of the rod moves within the sight glass in accordance with movement of the float and indicates the rate of fluid flow on a calibrated scale that is mounted on the guard adjacent the sight glass.
When an electronic feedback signal is required, a sensor may be used to determine the position of the indicator. Known devices typically utilize an optical design. An optical design may include a row of optical sensors on one side of the float rod and a row of light emitting diodes (LEDs) on the opposite side. The position of the float rod determines which optical sensors receive light. The optical sensors sense the presence or lack of light and provide an electrical signal in proportion to the flow rate.
A typical sight glass tube contains a fluid to not only aid in dampening but also to provide a media for the light from the LEDs. When the flow meter is used with dirty gasses, the dampening fluid is susceptible to contamination which degrades sensor performance to the point of inoperability. Sensor integrity is maintained by replacing the contaminated dampening fluid with fresh fluid. This process is time consuming and requires the equipment that the flow meter is servicing be down for maintenance. This downtime can result in loss of production which may further have a negative impact on the end user. Consequently, improved methods and devices for sensing indicator position are needed.